Carbon steel rods for constructing machines under severe conditions, alloy steel rods containing such special elements as Ni, Cr and Mo, and spring steel rods, etc. are normally subjected to various heat treatments before or during subsequent processing to end products. This invention relates to an apparatus for manufacturing softened wire rods from a hot rod mill with a view of eliminating one of the heat treatments, e.g. annealing and normalizing.
It is well-known to form the hot-rolled rod into non-concentrically overlapped rings and deposit it in this form onto a conveyor, and then rapidly cool the rings by forced air so they move to the delivery end of the conveyor where they are gathered into a bundle. This conventional rapid cooling is used for plain carbon steel rods containing low, medium and high carbon content, which are drawn and fabricated into end products without requiring further heat treatments. But this method is inapplicable to some alloy and carbon steels, especially for cold heading, which do not attain the desired quality unless they are cooled more slowly during allotropic transformation. The softening of high-grade steel rods especially calls for much slower and strictly controlled cooling. The targeted quality level cannot be attained unless such steels are cooled along a predetermined cooling curve.
U.S. Pat. No. 3,930,900 discloses an inline rod cooling method and apparatus. According to this publication, a laying head delivers hot-rolled rod in overlapped nonconcentric rings onto a conveyor. In order to cool the traveling rod rings uniformly, this publication employs a combination of the following three steps: (1) sending varying intensities of radiant heat in an amount substantially inversely proportional to the distribution of accumulated rod mass in the cross-section of the overlapped rings of the coiled rod per unit width of the conveyor to different parts of the rings across the width of the conveyor; (2) causing radiant energy to emanate from the portions of the coiled rings on both sides of the conveyor and restraining the emanation of heat from the middle thereof substantially according to the distribution of accumulated rod mass in the cross-section of the coil; and (3) minimizing the cooling of the rod due to convection by conveying the rings in an enclosed space with a controlled environment. The cooling apparatus for implementing this method comprises the combination of: a conveyor for forwarding the overlapped rings; a cooling chamber substantially covering the conveyor and the rings traveling thereon, the inside walls of the cooling chamber reflecting the radiant heat from the rings, and having a fixed base and a selectively movable top cover, an adjustable opening being provided in the side wall of the cooling chamber; and a radiation controller provided inside the cooling chamber facing the conveyor and spaced therefrom and having a plurality of radiating surfaces which are individually maintained at an independently pre-selected temperature by a plurality of independent temperature controllers. The object of this prior art system is to provide accurately controlled slow cooling along the entire length and also across the cross-section of the coils of the rod, which easily permits conversion to rapid cooling and cooling rate adjustment within the 0.degree. C./sec. to 20 C./sec. range.
As can be understood, the technique disclosed in the United States patent publication accomplishes cooling rate adjustment by selectively controlling not convection but radiation. In more concrete terms, this prior art technique takes into account the distribution of the rod mass per unit width of the conveyor on which the offset rings are laid. The technique comprises either applying radiant heat to the rod rings in substantially inverse proportion to the mass distribution, or causing radiant energy to emanate and restraining the emanation substantially according to the mass distribution. The rod mass in the cross-section is maximal at both sides of the conveyor where the rings overlap each other and minimal at the center of the conveyor where the rings are separate from each other. Accordingly, the rings release more of their heat at the center of the conveyor than at both sides. Therefore, if the rings are allowed to release heat naturally, i.e. without any regulating means the portions of the rings at the center of the conveyor cool off faster than the portions at the sides of the conveyor. The prior art publication considers that the desired effect can be obtained because irregular cooling of the rings is avoided by the control of radiation rates at different parts of the rings.
But studies and experiments made by the inventors have shown that the understanding of those in the prior art is not altogether correct. Rather, the method of the prior art has proved to be incapable of completely eliminating the irregular cooling at different parts of the rings. Controlled cooling according to the prior art has also proved ineffective, particularly in obtaining the desired mechanical properties for high-grade steel rods which meet difficulty in softening.
In their studies, the inventors measured the temperatures at different points, as indicated by the symbols in FIG. 1, on the outer surface area and the center area of the cross-section of the coil of overlapped rings of the coiled rod traveling along a roller conveyor in a cooling chamber enclosing the coiled rod and roller conveyor for controlling the environment. The temperatures at the different points were measured at several points along the conveyor, i.e. after different holding times. The results obtained are shown in FIG. 2. For the purpose of making these measurements, the coiled rod was heated to a temperature substantially equivalent to that at which the hot-rolled rod is actually delivered from the laying reel, and the temperature of the atmosphere in the upper part of the cooling chamber was kept at 650.degree. C. to impede convection heat loss from the coiled rod. As is evident from FIG. 2, the temperature profile across the width W of the cross-section of coiled rings is higher at the two edges than at the center, and the difference is in proportion to the distribution of the rod mass. Overall, the temperature is highest at the middle of the vertical dimension of the two edges of the cross-section where the rod mass (or density) is great and lowest at the bottom where the rings in the cross-section contact the roller conveyor. That is, the greatest temperature difference, exceeding 100.degree. C., exists between the core (center) and bottom of the two edge portions of the cross-section where the rod mass concentration is maximal. Presumably, this is due to the fact that the core portions of the two edges of the cross-section are held at high temperatures by the heat carried by the rod from the preceding hot-rolling process, the least amount of heat being released from these portions due to the heaviest rod mass concentration. Meanwhile, the bottom surface portion of the cross-section contacts the rollers of the conveyor. To prevent thermal wear, the bearing units of each roller are provided outside the cooling chamber, so that the bottom surfaces of edges of the cross-section located close to the bearing unit are cooled the most, releasing the greatest amount of heat by heat conduction through table rollers.
As will be understood, applying radiant heat in inverse proportion to the rod mass distribution across the width W of the cross-section or causing release of radiant energy from the two edges of the cross-section according to the rod mass concentration and restraining the release of heat from the middle portion, as proposed in U.S. Pat. No. 3,930,900 will not eliminate the temperature difference between the core portions of the two edge portions and the bottom surfaces of the cross-section and, therefore, as a result, will cause the non-uniform cooling. In practice the prior art method actually accelerates the supercooling of the bottom surfaces of the cross-section.
When the hot-rolled rod is transferred onto a conveyor, the coil still retains a considerable amount of heat which can be effectively utilized for softening in the cooling chamber, permitting considerable energy saving. But the cooling chamber according to the above-described prior art does not make effective use of the heat retained by the rod, but rather supplies radiant heat from a radiant tube (or a radiant heat controller) provided therein.
From the foregoing and from the results of various experiments, the inventors have found the following:
(1) From the viewpoint of equipment layout and investment cost, it is advantageous to perform in-line slow cooling of the hot-rolled rod in the shortest possible time and on the shortest possible line. It is therefore desirable to pack the coiled rings on the conveyor as densely as possible.
(2) It is necessary to carry out controlled cooling to minimize the temperature difference among the different parts of the cross-section being slow-cooled and thereby cool the entire coil uniformly.
(3) To achieve energy saving, the heat carried over from the hot rolling process must be effectively utilized for the slow-cooling.
(4) To make it possible to use a limited treating time and line length, steels that are difficult to soften must be cooled in a highly efficient manner using close temperature control to cool them according to a preestablished cooling curve.
(5) It is desirable that cooling equipment be capable of performing not only the above-described slow-cooling but also conventional forced-air cooling properly and rapidly so as to be usable for a variety of steels.
In conventional forced-air cooling, the rod is cooled rapidly. This rapid cooling following the rolling produces uniform, fine pearlitic structures in high-carbon steel rods, imparting good drawability. In rods of plain carbon and alloy steels for machine structural use, however, the formation of fine pearlite by rapid cooling is not altogether desirable for subsequent processing. In order to give these steels a perfect ferrite-pearlite structure and soften them to the desired degree, they must, on the contrary be cooled slowly at a rate of not higher than approximately 0.2.degree. C./sec.